Inorganic scintillation detectors convert incoming radiation (alpha, beta, or gamma) into pulses of light (optical photons). The amplitude (and frequency) of the pulses of light corresponds directly to the energy imparted by the incident radiation. Inorganic scintillation detectors (Nal(Tl), Csl(Tl), Csl(Na), BGO, LaCl3, and the like) have been used for many years in gamma ray spectrometry and have numerous commercial applications. Inorganic scintillation detectors, as traditionally understood, exhibit strong temperature dependence and corresponding count rate limitations.
FIG. 1 shows plots comparing the temperature dependencies for different inorganic scintillators. The temperature dependence exhibited by inorganic scintillation detectors is nonlinear, and, thus, it is difficult to compensate accurately over a given temperature range. FIG. 2 shows the dependence of the scintillation light decay constant exhibited by Nal(Tl) due to the temperature of the Nal(Tl) crystal. Again, it is apparent that the light decay constant is nonlinear with respect to the varying temperature of the crystal.
There are actually four temperature factors that affect the spectrum (gain) dependence of inorganic scintillation detectors. The four factors are: [1] temperature coefficients of the photomultiplier tube (PMT) gain and photo-cathode sensitivity; [2] temperature dependence of the scintillator light yield; [3] temperature dependence of the scintillator decay time constant; and [4] the characteristic of the signal shaper that is used in the electronics package. Thus, a model that can accounts for all four-temperature factors is more accurate than a model with a single temperature-dependent time constant.
There are four historical methods recognized to address the temperature effects on gain stabilization:
First, scintillators can be stabilized electronically by introducing an americium-241 seed and tracking the alpha peak and adjusting the gain of gamma peaks. However, the temperature response of alpha particles is different than for electrons generated by gamma-ray interactions, so additional temperature compensation is still required.
Second, mapping the temperature versus gain (FIG. 1) for the particular inorganic detector used and then offsetting the resultant output accordingly. In this method, a temperature sensor is attached to the scintillation material and the reading is used to adjust the gain. The problem of this approach is that the temperature versus gain curve is a function of both the pulse shaping time constant and the doping level of the inorganic scintillator. Doping is defined as the addition of a small amount of an impurity (e.g. thallium in a Nal crystal or cerium in a LaCl3 crystal) to boost the light output from the scintillation material (crystal).
Third, use a temperature dependent photomultiplier tube divider as taught in U.S. Pat. No. 6,407,390 issued on Jun. 18, 2002 by Csaba M. Rozsa. This method extends the scintillation detector operating range down to ambient temperatures above 0° C. by using linear segmented approximations with a fixed gain assigned to each linear segment. Note this method works only for ambient temperatures above 0° C.
The fourth method involves the use of a light emitting diode (LED). By tracking the constant light reference pulse produced by a LED, the gain of the scintillation detector is stabilized. However, this approach is applicable only if the LED light pulse follows the amplitude and shape changes of the light pulse produced by the scintillation detector due to changes in the ambient temperature. Thus, this method does not work in the entire temperature range (−30° C. to +60° C.), but only for temperatures around the maximum of the temperature characteristic as displayed in FIG. 1.
Unlike the approaches described above, the apparatus and method of the present invention provides a novel and optimum solution for addressing all four factors that affect the spectrum gain dependence of inorganic scintillation detectors due to temperature.
The apparatus and method of the present invention addresses the problems incurred by temperature dependence and count rate limitations, and offers higher practical counting rates when used with conventional scintillation materials. Up to 5 fold improvements in dynamic range without significant resolution degradation (less than 15% at 662 keV for up to 800K throughput cps) or peak shift [less than 2% shift for up to 800K throughput cps] are made possible by applying the present invention to scintillation detectors. Devices employing the present invention may operate over a wide temperature range (−30° C. to 60° C.) without use of an external radiation source for temperature stabilization.
The present invention will improve the performance of many well-known particle-probe devices such as CAT scanners by providing an expanded count rate (greater than a factor of 5). The expanded count rate is a consequence of the step response exhibited by the preamplifier used in the present invention to the scintillator's light pulse. The step response eliminates the “pile-up” effects due to slow components of the light pulse and related deterioration of the light spectra at higher count rates.
By expanding the count rate, patient exposure time can be reduced, or the counting statistic may be increased, by the same factor. An expanded counting range also allows portal detectors to differentiate between patients who have recently undergone radiological treatments and industrial shipments with substantial radioactive sources (e.g., soil density monitors).
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.